Page 497 - Water and wastewater engineering

12-14 WATER AND WASTEWATER ENGINEERING

Chlorine
solution
reservoir
Calibration
column

Positive
displacement Backwash
pump pump
Flow Backwash
P1 P1
meter valve
Water FM
supply Feed Filtrate
Feed Strainer valve valve
pump Feed Membrane Filtrate
Backwash sample modules sample
discharge point point
Distribution
Filtered
water
reservoir
FIGURE 12-8
Schematic of typical pressure MF/UF facility.
( Source: U.S. EPA Membrane Guidance Manual, 2005.)

particle removal efficiency) and others reporting increased fouling. It appears that if the design of
the coagulation system (including sedimentation) is to remove a significant fraction of the NOM
(15 to 50 percent), MF membrane performance will be improved, but little improvement may be
expected for UF membranes (Howe and Clark, 2006). The effect of coagulation is site specific
due to the interactions between the coagulants, raw water components, and the membrane materi-
als. In some cases, low doses may cause greater fouling than no coagulation, but higher coagulant
doses for enhanced coagulation (for example, 25 to 50 mg/L of alum) frequently reduce fouling.
In any event, performing pilot plant studies over a period of time to examine a variety of raw
water conditions is recommended (Bergman, 2005; MWH, 2005).
Where iron and manganese are prevalent in the raw water, oxidation may be performed to
form a precipitate that can be removed before the membrane treatment step. The common oxi-
dants are chlorine, chlorine dioxide, ozone, and potassium permanganate. The use of oxidants
requires careful selection of the membrane material, as well as precautions to remove excess
residual oxidant before the membrane treatment step.
The minimum pretreatment requirements are established by the membrane manufacturer. In
general, they will include strainers or bag filters rated in the 50 to 500 m size range to protect
the membrane from excessive solids loading (AWWA, 2005; Bergman, 2005).

Concentrate Stream
The primary concern in disposing the concentrate stream is suspended solids. Disposal methods
include land application, discharge to the municipal sewer system, ocean discharge, and deep
well injection. Disposal of the concentrate is a major issue in the selection of this technology and
should be addressed early in the design process.